The invention relates to a storage rack arrangement for the storage of nuclear fuel elements.
Nuclear fuel elements and in particular spent nuclear fuel elements are stored in storage pools which are filled with a coolant such as water which simultaneously serves as a shield against the radiation of the fuel elements. The spent nuclear fuel elements remain in the storage pool for so long until they are supplied for reprocessing or, on a case by case basis, final disposal. Storage racks have been developed for the safe storage of nuclear fuel elements, wherein a storage pool can accept a plurality of storage racks which can be arranged side by side and, on a case by case basis, also above one another.
A storage rack is known from document DE 29 30 237 A1 for the storage of nuclear fuel elements which contains a plurality of vertical shafts or channels for the reception of the fuel elements, with the walls of the chancels being formed from sheet metal strips arranged above one another. The sheet metal strips are provided with incisions at their lower and upper margins by means of which the sheet metal strips pushed into one another cross-wise are mutually held. The sheet metal strips are arranged between an upper grid plate and a lower grid plate which are connected by braces. The channels and the fuel elements are supported on the lower grid plate in the described storage rack.
A further storage rack for the storage of nuclear fuel elements in a storage pool is described in the document U.S. Pat. No. 4,042,828. The storage rack contains a plurality of upright enclosures arranged in rows and forming channels for the reception of the fuel elements. The enclosures which have a square cross-section are arranged in an open rack frame and are open at the lower and upper ends so that the water contained in the storage pool can circulate in the enclosures to cool the fuel elements. The fuel elements placed into the enclosures are supported individually on the floor of the storage pool via supports.
It has been found that the above-described storage racks from the prior art only satisfy the demands of a safe storage of the fuel elements with reservations in that noticeable displacements of the storage racks in the storage pools are in particular determined during earthquakes. A storage rack fully loaded with fuel elements typically has a weight between 30 tons and 60 tons so that the forces which occur during an earthquake between adjacent storage racks and between the storage racks and the walls of the storage pool are substantial and there is the risk that the storage racks and the fuel elements stored therein and/or the lining of the storage pool may be damaged during an earthquake and that radionuclides may be released in this process.
FIGS. 9, 13 and 14 from the aforesaid document U.S. Pat. No. 4,042,828 show holding clamps by means of which storage racks disposed next to one another can be connected. A displacement between the storage racks during an earthquake can be avoided at least in principle by a connection of the storage racks. The manner of construction of the storage racks described in the named document, however, appears unsuitable to take up larger horizontal forces so that the security against earthquakes can only be increased with limitations by means of the holding clamps.
To get to grips with the problem of the earthquake-induced storage rack displacements and the risk associated therewith of the collision with the walls of the storage pool or other apparatus installed in the storage pool, attempts have been made to support the storage racks at the walls of the storage pool and/or to anchor them or screw them tight to the floor of the storage pool. In the past, a plurality of storage pools have been built which are based on this fixed storage rack principle. It was, however, quickly found that this fixed storage rack principle is subject to limitations. At higher earthquake loads and at a higher storage density or at higher masses of the storage racks, locally extremely high peak loads arise at the anchorage points or support points which would necessarily result in damage to these structures, in particular also in leaks in the liner of the storage pool. For this reason, a solution was sought which is suitable for higher earthquake loads and higher storage densities.
A substantial improvement for this problem is provided by the so-called “free-sliding” principle which has established itself in newly built fuel element stores in the past few years. In this principle, the storage racks are installed freely in the storage pool and can move in a free-sliding manner in an earthquake. A large portion of the seismic energy is destroyed by the friction occurring on the movement. This principle allows the elimination of the locally extremely high peak loads at the anchorage points or support points of the storage racks, but also has specific disadvantages.                One disadvantage is that the earthquake-induced storage rack displacements require a certain free zone all around the storage racks. This is equal to a loss of storage area, which is very expensive.        A further disadvantage is that these displacements result in the formation of irregular spacings of different magnitudes between the support racks and that an irregular arrangement of the storage racks can thereby arise after an earthquake which can result in problems with the later handling of the stored fuel elements.        A further disadvantage results from the fact that storage racks which are loaded partly and in particular unilaterally are excited to sway at high horizontal earthquake accelerations, with the feet of the storage racks, for example, being able to lift 10 to 20 mm from the floor of the storage pool. Such a sway of the storage racks generates very high impact forces on the floor of the storage pool, whereby the risk of a leak of the storage pool is substantially increased. In addition, an increased tendency to storage rack displacement in the direction of the unloaded site is observed which additionally degrades the stability 20 of the storage racks.        